The Federal Communications Commission (FCC) controls broadcasting rules for the United States, specifically including the properties of broadcast signals for radio and television, in coordination with the International Telecommunications Union (ITU). For television, broadcast emission is limited to a single predominant linear polarization and a single predominant circular polarization. For audio broadcasting (radio), a Very High Frequency (VHF) band from 88 MHz to 108 MHz is assigned for transmission of (analog) Frequency Modulated (FM) signals. The band, with reference to its frequency range rather than any specific modulation technology, is referred to herein as the FM band. “Channels,” as referred to herein, are the one hundred channels, centered at 200 KHz intervals, specified by the FCC, wherein modulation of +/−75 KHz is defined as 100% modulation, wherein output deviating from the center frequency by +/−120 KHz to +/−240 KHz is required to be 25 dB below the level of the unmodulated carrier, and wherein output deviating from the center frequency by +/−240 KHz to +/−600 KHz is required to be 35 dB below the level of the unmodulated carrier. As these requirements make evident, gaps between channels are controlled by modulator and filter rolloff rather than by assignment of forbidden zones.
Broadcasters in the FM band are permitted to radiate with horizontal (linear) as well as left-hand and right-hand circular/elliptical polarization (FCC regulations, 47 CFR §73.316 et seq.). The Medium Frequency (MF) broadcast band from 535 KHz-1605 KHz uses Amplitude Modulated (AM) signals, and is referred to herein as the AM band, again with reference to frequency rather than modulation technology. AM radio uses somewhat different rules and is not addressed by this invention.
FIG. 1 shows a spectrum mask 10 of allowable power versus frequency for a broadcast channel, wherein a center frequency F0 12 is one of the FCC-defined FM-band analog channel center frequencies, and lower and upper frequency mask 10 limits FLO 14 and FHI 16 represent the −25 dB extremes described above. Signal strength of a realizable analog FM transmitter will ordinarily have an envelope 18 of power as a function of deviation from the center frequency. To each side of the first threshold of the spectrum mask 10 are groups 20 made up of multiple digital subchannels 22. These fall within FCC regulations, and contain the digital portion of IBOC® broadcasting. Total energy in each group 20 is specified to be 20 dB below the total analog envelope energy.
IBOC® is transmission of a digital signal or of an analog signal and a digital signal simultaneously on a single assigned channel within the AM or FM band. The signal in the analog envelope 18 in the FM band is frequency modulated. The lower and upper digital subchannels 20 are orthogonal frequency division multiplexed (OFDM) data streams that may include information such as the audible content of the analog FM signal, channel pilot tones, ancillary data such as program text, and such other information as a broadcaster may choose to transmit. The digital subchannels 22 contain appreciable energy only outside the −25 dB limits of the analog energy mask 10 specified for the channel.
IBOC® digital signal energy falls generally within the bandwidth of FM band analog broadcast antennas. It is possible to cobroadcast the analog and digital content using a single transmitter, transmission line, and antenna, but may be difficult for multiple reasons, including the bandwidth of existing high-power (vacuum tube) analog-only transmitters and the power output of existing (solid state) wide-bandwidth transmitters. Strategies for circumventing these limitations include combining the output of multiple (lower power) cobroadcasting-capable transmitters, combining separate analog transmitter and digital transmitter output signals, and numerous others.
Licensing of broadcasting is restrictive, with rules defining signal bandwidth and purity (out-of-channel and other harmonic energy), signal strength as a function of distance from a broadcast antenna, direction of emission, height from which emission occurs, and the like, as well as content.
Antennas can be single dipoles or any other styles that satisfy regulations and meet broadcasters' requirements. Many antennas are composed of multiple radiating elements, with each element or group of elements occupying a so-called bay, that is, a vertical location along an antenna tower, with the bays spaced apart by distances that may approximate a half-wavelength or one wavelength of the signal center frequency for which the antenna is designed. An antenna can be defined as an assemblage that includes a number of bays distributed over an aperture, wherein the aperture as used herein is the distance from the topmost to the bottommost extent of the radiating elements. One effect of using an extended aperture, realized in some embodiments with multiple bays, is to increase gain, that is, to reinforce emission in a main beam in the shape of a flattened torus surrounding the antenna (uniformly if omnidirectional) and to partially cancel and thus suppress emission above and below the main beam. The main beam can be deflected toward or away from the ground by adjusting interbay spacing as compare to the nominal half- or full-wavelength spacing, a principle termed beam tilt. Broadband antennas, defined herein as those which can emit efficiently for several channels, have interbay spacing selected for a particular frequency (in effect, a single channel) within the antenna bandwidth, with other channels typically exhibiting somewhat reduced gain and different beam tilt.
Antennas can achieve output signal polarization by structure and orientation of elements and by interaction of elements. For example, a single, vertically oriented, free-standing, center-driven dipole emits, by default, a vertically polarized, omnidirectional signal with strength approximately toroidal with azimuth and elevation. By contrast, a vertical slot antenna center-driven between the edges of the slot emits a horizontally polarized signal, generally in a single predominant azimuthal direction, such as with a skull or cardioid pattern of signal strength in both azimuth and elevation.
A circularly polarized signal can, like a linearly polarized signal, be emitted in multiple ways. (Note: circular polarization (CP) is the limit of elliptical polarization, at which limit signal magnitude is substantially equal at all angles. As used herein, CP is a shorthand term for all rotating polarizations. Ghost rejection, like the characteristic 3 dB gain reduction from use of a linearly polarized receiving antenna and the jagged boundary of magnitude with angle, is an attribute of CP broadcasting not further addressed herein.) Ways for emitting CP include forcing a signal to propagate with CP by exciting two or more radiators with the same signal, but with different phase delay, which can produce CP as measured at far field. Antenna elements designed for this can be electrically symmetrical, permitting the phase of the applied signals to determine whether the emitted signal is linearly polarized or is left- or right-hand-circularly polarized. Thus, in particular, by splitting a signal, delaying half of it for a specific time, and applying it to specifically-oriented and -spaced components of an antenna element, a first circular polarization can be achieved, while an equivalent signal, split similarly but delayed oppositely, can achieve opposite circular polarization simultaneously from the same antenna element.
As shown schematically in FIG. 2, one prior-art approach to combining analog and digital FM band signals for IBOC® has been to feed analog 24 into one input port 26 of a 3 dB hybrid 28 and digital 30 into the opposite input port 32. As is well known in the art, a transmission line-compatible hybrid 28 of the type shown, variously known as a 3 dB, 90 degree, or quarter-wave coupler or hybrid, accepts one or two input signals on ports assigned as input ports 26 and 32, respectively, and emits output signals on the remaining two ports 34 and 36. The split signals on the respective output ports 34 and 36 are phased in such a way that a suitable antenna element, such as the crossed dipole pair 38 in FIG. 2, emits in the out-of-the page direction with right hand polarization 40 for one signal and left hand polarization 42 for the other signal.
As shown in FIG. 3, it is well known in the art that when dipole radiators are placed in an orientation other than orthogonal to one another, they will mutually couple energy, a process that increases with proximity as well as with the extent of parallelism. Each dipole serves as both a transmitter and a receiver for energy to and from the other dipole. It follows that when coplanar dipoles 44, 46 are placed in a crossed configuration, having an angular difference θ, they will exhibit cross coupling in proportion to the extent to which the dipoles are nonorthogonal, that is, that the angle θ differs from 90 degrees. Thus, cross coupling in FIG. 3 increases with a first polarity as θ decreases from 90 degrees toward zero, and increases with a second polarity, opposite to the first polarity, as θ increases from 90 degrees toward 180 degrees. If the dipoles 44, 46 are noncoplanar, the effect is diminished but not eliminated.
As shown in FIG. 4, and as is well known in the art, it further follows that when crossed dipole elements 44, 46 are part of a vertical array of such elements 48, mutual coupling will occur from bay 50 to bay 50 in proportion to the closeness of interbay spacing and the parallelism of corresponding dipoles in the respective bays 50.
As shown in FIGS. 5 and 6, and as is well known in the art, the effects of mutual and cross coupling can be seen in the antenna's overall impedance, with the magnitude and the phase of intercomponent coupling changing the input impedance of each dipole 44, 46. A single pair of crossed dipoles in free space, driven from a hybrid 28, as shown in FIG. 5, has characteristic impedance that can be represented on the respective ports' Smith charts 52 and 54.
For two pairs of crossed dipoles, as shown in FIG. 6, interaction between the corresponding dipoles in the pairs becomes a factor. If the output ports 34, 36 of the respective hybrids 28 feed the respective dipoles 44, 46, and if the phase delay from the respective analog input ports 26 to the output ports 34, 36 is opposite to the phase delay from the respective digital input ports 32 to the output ports 34, 36, as would be expected with a typical 3 dB hybrid 28, then there is a 90 degree phase difference in the mutual coupling effect between the dipoles 44, 46. This in turn causes a difference in input impedance between the dipoles 44, 46, creating a shift in input impedance at the input ports of the respective hybrids 28. This difference is represented in FIG. 6 by the difference between the respective ports' Smith charts 52 and 54.
FIG. 7 shows an antenna 60 having two sets of crossed dipoles 62 and 64, respectively, fed by hybrids 66 and 68, respectively, wherein the lower hybrid 66 is shown as driven and the upper hybrid 68 is examined for its properties. As shown in FIG. 7, corrections can be made to one set of dipoles 56, 58—here, dipoles 56 are lengthened—to compensate for the impedance shift into one of the hybrid input ports of the upper hybrid 68. In this example, the upper input port 38 is successfully compensated by the lengthened dipoles, despite the presence of the lower element 62. However, this compensation comes at the expense of making the match worse into the lower input port 32. In the prior art, this practice was common and practical, since only one of the two input ports 32 or 38 was usable. In FIG. 7, respective dipoles 56 of the elements 62 and 64 are made longer in order to make their impedance in the presence of mutual coupling the same as the impedance of an equal-length set of dipoles in free space. As shown, this compensation technique only works with respect to one input port 38 of the affected hybrid 68. The opposite input port 32 exhibits twice the shift in input impedance, with that shift in the opposite direction, which only matters if the other input port 32 is used. Thus, this type of antenna cannot be used as a dual input design unless the radiating elements are symmetrical, which requires that both mutual and cross coupling be eliminated; as derived here, this is clearly infeasible with the simple dipoles shown in FIG. 7.
The radio industry and the FCC have standardized on the iBiquity® IBOC® hybrid analog-digital transmission system. FM stations in the U.S. are permitted to simultaneously broadcast analog and digital signals within their current allocated frequency range. One method of achieving the simulcast is to use two separate transmission systems driving two separate antennas, with the antennas isolated sufficiently, such as by spatial separation, to produce minimal interaction. Another simulcasting method uses a hybrid-fed, crossed-dipole configuration, wherein the analog and digital signals are fed into the zero- and 90-degree ports of the hybrid, producing right-hand analog and left-hand digital polarization from the single antenna. U.S. Pat. No. 6,934,514 discloses an embodiment of this method. This method inherently includes cross coupling between dipole components within each element and mutual coupling between corresponding components in different antenna bays. With existing designs, the compensation required to neutralize the coupling into one hybrid input port adversely affects the opposite input port, so that a good match cannot be achieved into both input ports simultaneously. This can limit performance of this design in a dual-input antenna configuration.